The present invention relates to a magnetic recording/playback apparatus for recording and/or playing back digital video signals, and particularly to the recorded data arrangement and the playback of this recorded data in order to obtain good quality high speed playback pictures.
Accompanying the trend in recent years toward large screen size for consumer color television receivers, progress has continued toward higher picture quality in video signal recording/playback media.
Also, as storage media for recording and playing back high quality images, many firms are developing a digital magnetic recording/playback apparatus (referred to below as digital VTR) for consumer use wherein the video signal is digitized and the band compressed (high-efficiency encoded) when performing recording and/or playback.
A prior art digital VTR recording/playback system for consumer use is described in the IEEE Transactions on Consumer Electronics, Vol. 34, No. 3, Pages 597 to 605 titled "An Experimental Digital VCR with 40 mm Drum, Single Actuator and DCT-based Bit-rate Reduction" relating to a recording/playback system of Philips.
The block diagram of the recording system of a prior art consumer use digital VTR is shown in FIG. 41. As illustrated it is provided with an input 1a for the luminance signals Y and two additional inputs 1b and 1c for chrominance signals CR and CB. A/D converters 2a to 2c are used to convert signals from analog to digital form. A high-efficiency encoder 3 performs high-efficiency encoding of the input luminance signal Y and chrominance signals CR and CB. An error correction encoder 4 adds, to the outputs of the high-efficiency encoder 3, an error detection code used for correcting or detecting errors generated during playback. Digital modulators 5a and 5b apply digital modulation to the data outputs from the error correction encoder 4. Sync adders 6a and 6b add synchronization and ID signals. Further provided are recording amplifiers 7a and 7b to amplify the output of the sync adders 6a and 6b, and rotary heads 8a and 8b are used for magnetic recording and/or playback on a magnetic tape 9.
The block diagram of the playback system of a prior art consumer use digital VTR is shown in FIG. 42. As illustrated, it is provided with members 8 and 9 which are the same as those with identical referenee numerals. Head amplifiers 10a and 10b amplify the signals played back by the rotary heads 8a and 8b. Data detectors 11a and 11b detect the data from the playback signals and detect and correct jitter in the playback signals. Digital demodulators 12a and 12b demodulate the outputs of the data detectors 11a and 11b. An error correction decoder 13 detects and corrects errors in the playback signal. A high-efficiency decoder 14 performs high-efficiency decoding of the error correction decoder 13 for restoring the video signal. D/A converters 15a to 15e convert digital signals into analog form. The outputs of the D/A converters 15a to 15c are output via output terminals 16a to 16c.
FIG. 43 shows a block diagram of a prior art high-efficiency encoder 3 provided in a magnetic recording/playback apparatus. As shown in the the figure, this high-efficiency encoder 3 comprises field memories 17a and 17b, a DCT circuit 18, an adaptive quantizer 19, a variable length encoder 20, a buffer memory 21, and a buffer controller 22. The DCT circuit 18 performs two-dimensional discrete cosine transform (two-dimensional DCT) with respect to block data from field memories 17a and 17b.
An adaptive quantizer 24 quantizes the coefficients obtained by transformation by the DCT circuit. The variable length encoder 20 performs variable length encoding of the output of the adaptive quantizer 19. The buffer memory 21 is used for converting the output of the variable length encoder 20 into a fixed rate output. The buffer controller 22 changes the quantizing parameters of the adaptive quantizer 19 in order to avoid overflow of the buffer memory 21 and selects the components which are to be encoded by the variable length encoder 20.
FIG. 44 shows a high-efficiency decoder 14 provided in the prior art magnetic recording/playback apparatus. As illustrated, the high-efficiency decoder 14 comprises a variable length decoder 23, a buffer memory 24, an inverse adaptive quantizer 25, an inverse DCT circuit, and field memories 27a and 27b.
The variable length decoder 23 converts the variable length encoded data into the original data of the fixed length. The buffer memory 24 provides the data from the variable length decoder 23 at a fixed rate. The inverse DCT circuit 26 (IDCT) performs inverse discrete cosine transform on the data output from the inverse adaptive quantizer 25. Field memories 27a and 27b delay the playback digital signal output from the IDCT 26 by a predetermined amount and decode the blocks formed at the time of recording.
FIG. 45 shows a block diagram of a drum motor and capstan motor control system. As illustrated, this capstan motor control system comprises a motor driver 51, a drum motor controller 52, a control head 53, a capstan controller 54, and a capstan driver 55.
The motor driver 51 produces voltage for driving the drum motor 50. The drum motor controller 52 performs drum motor control on the basis of a reference signal supplied from a central controller 49, and drum PG and FG outputs from the drum motor 50.
The control head 53 reads the control (CTL) signal from the control track of the magnetic tape 9. The capstan motor driver 55 produces the voltage for driving the capstan motor 56. The capstan motor controller 54 uses the CTL signals from the drum motor controller 52 and the control head 53 for controlling the capstan motor 56.
The central controller 49 performs overall control of the digital VTR. For example, in order to perform recording, normal playback and high speed playback in accordance with a manually activated input signal at the manual input section 48, the control signals are sent to the various VTR sections. When selecting normal or high speed playback, the normal or high speed playback instruction signal, and, for a VTR model provided with a plurality of high speed playback speeds, the signal for selecting a desired speed from among these, is applied to the drum motor controller 52 and the capstan controller 54.
Following is a description of the recording format of this digital VTR. FIGS. 46A and 46B are schematic diagrams showing a magnetic tape and drum arrangement relationship and a track pattern formed on the magnetic tape 9 of a prior art magnetic recording/playback apparatus, wherein two-channel combination beads 8a and 8b are utilized for the rotary heads for the respective channels.
FIG. 46A shows the arrangement of two-channel rotary heads on the rotary head drum. The type of arrangement shown in the figure is used for two-channel combination heads. FIG. 46B shows the recording track pattern formed on the magnetic tape 9.
As illustrated, the two-channel rotary heads 8a and 8b are arranged adjacent to each other in a rotary drum 60. During recording, the above-mentioned two-channel recording signals are recorded substantially simultaneously on the magnetic tape 9 by the rotary heads 8a and 8b (see FIG. 46B).
It is customary to refer to the rotary head 8a as the channel A (CH--A in the following) rotary head and the rotary head 8b as the channel B (CH--B in the following) rotary head. The CH--A rotary head 8a and CH--B rotary head 8b possess mutually different azimuth angles. In the figure, A and B indicate the respective recorded tracks formed by the different rotary heads for the respective channels. In prior art examples, the drum rotating speed is 9,000 rpm. Consequently, since two-channel recording is performed with a prior art example as mentioned above, as indicated in FIG. 46B, one field of video data is distributed among 5 tracks and thus, one frame of video data is distributed among 10 tracks.
Following is a description of the recording system operation with reference to FIG. 41.
The luminance signal Y and two chrominance signals CR and CB input via the input terminals 1a to 1c are converted to digital form by the A/D converters 2a to 2c, and the recording bit rate is reduced by the high-efficiency encoder 3. A detailed description of the high-efficiency encoder operation is given later. At the error correction encoder 4, an error correction (check) code is generated and added to the recording signal data for correcting and detecting errors produced during playback.
In accordance with a predetermined modulation method, the recording digital signal is applied to the digital modulators 5a and 5b, where the low frequency component of the recording signal is suppressed (digital modulation). Sync and ID signals are applied to the digitally modulated recording data by sync signal adders 6a and 6b. After amplifying by the recording amplifiers 7a and 7b, the signals are recorded on the tape 9 via the rotary heads 8a and 8b.
Similarly, following is a description of the playback system operation with reference to FIG. 42.
The two-channel signals played back from the tape 9 by the rotary heads 8a and 8b are amplified by the head amplifiers 10a and 10b. The data detectors 11a and 11b convert the signals into digital data and also absorb jitter (time base error) in the playback signal. The digital demodulators 12a and 12b demodulate the signals to form the playback digital signal, which enters-the error correction decoder 13.
At the error correction decoder 13, on the basis of the error correction code added beforehand at the time recording, errors produced in the playback signal are corrected or detected. The signal is then applied to the high-efficiency decoder 14 for such processing as variable length decoding and inverse DCT, after which the original luminance signal Y and two chrominance signals CR and CB are restored. These are converted to analog form by D/A converters 15a to 15c and sent out via the output terminals 16a to 16c.
Next is a description of the high-efficiency encoder 3 with reference to FIG. 43.
The input luminance signal Y and two chrominance signals CR and CB signals are delayed by a predetermined amount by the field memories 17a and 17b, then formed into blocks. In the block formation, the input signals are first divided among blocks of 8 picture elements by 8 lines. In the prior art example, the blocked luminance signal Y and two chrominance signals CR and CB are time-division multiplexed and applied to the DCT circuit 18, where discrete cosine transform is performed.
When the block picture element data is expressed as X (i, j) (i=0, 1, . . . 7; j=0, . . .7), the DCT circuit computes 8 DCT points in the horizontal direction as follows. ##EQU1##
Then 8 point DCT is performed in the vertical direction with respect to this transformed data f (m, j) (m=0, . . . , 7; j=0, . . . 7). ##EQU2##
This transform coefficients F (m, n) (m=0, . . . ,7; n=0, . . .7) are output.
The output transform coefficients from the DCT circuit 18 are quantized by the adaptive quantizer 19. The adaptive quantizer 19 possesses a plurality of quantizing tables with different quantizing steps. The quantizing steps are selected in accordance with the transform coefficients of each block and the parameters from the buffer memory 21. For example, a high contrast rising component is coarsely quantized, while a low amplitude detail component is finely quantized. The output of the adaptive quantizer 19 is variable length coded at the variable length encoder 20, and is then stored in the buffer memory 21.
The data stored in the buffer memory 21 are read out at a fixed rate. The buffer controller 22 detects the data stored in the buffer memory 21 and determines the quantizing parameters according to the data amount, and controls the adaptive quantizer 19. The buffer controller 22 also selects the encoding transform coefficients of the variable length encoder 20 from the amount of data amount in the buffer memory 21.
Similarly the operation of the high-efficiency decoder 14 is described below with reference to FIG. 44.
The playback digital signal output from the error correction decoder 13 is applied to the variable length decoder 23 for converting into fixed length data. This fixed length data is read out from the buffer memory 24 at a fixed rate, and is inverse-quantized at the inverse adaptive quantizer 25. It is then sent to the inverse DCT circuit 26, where inverse discrete cosine transform (IDCT) is applied to the input playback digital signal. The playback luminance signal Y and two chrominance signals CR and CB from the IDCT circuit 26 are temporarily stored in the field memories 27a and 27b, and delayed by predetermined amounts. Then, the block processing at the time of recording is decoded and the signals are sent to the D/A converters 15a to 15c.
Next is a description of the drum and capstan motor control system operation during playback with reference to FIG. 45.
During playback, the drum motor controller 52 uses an externally applied input reference signal and the drum PG and FG signal outputs from the drum motor 50 to control rotation of the drum motor 50. The drum controller 52 also produces the reference signal for capstan control during playback. On the basis of the control signal output from the drum motor controller 52, the drum motor driver 51 produces the voltage for driving the drum motor 50.
At the capstan controller 54, the CTL signal output from the control head 53 is used for controlling the tape transport speed, and the reference signal output from the drum motor controller 52 and the CTL signal are used for controlling the phase of the rotary heads 8a and 8b. The capstan driver 55 produces the voltage for driving the capstan motor 56 on the basis of the control signal output from the capstan controller 54.
Following is a description of the high speed playback operation with the above mentioned digital VTR. As a starting point, investigations using the actual equipment for determining the amount of data obtainable during high speed playback with a digital VTR are described.
The configuration of the error correction codes of the digital VTR used in the investigations is shown in FIG. 47. The code in the recording direction (referred to below as code C1) is a (241,225, 17) Reed-Solomon code, and the code in the vertical direction (referred to below as code C2) is a (116, 108, 9) Reed-Solomon code. As the recording format, one error correction block composed of the product code format is recorded on one-track.
In the measurement for the studies with the actual equipment, since the track data of the playback-signal from the rotary heads 8a and 8b is played back intermittently as shown in FIG. 50B, error correction was performed only by the C1 code and not by the C2 code.
The results of the experiment are illustrated in FIGS. 48 and 49. The results of error correction by the code C1 at five-time speed playback are shown in FIG. 48 and for eight-time speed playback are shown in FIG. 49. In this prior art example, code C1 is a Reed-Solomon code with minimum distance 17 and capable of correcting a maximum of 8 errors. In FIGS. 48 and 49, the horizontal axes indicate the corrected error data incidents and the vertical axes indicate their occurring frequency. For example, at five-time speed, the data quantity where the corrected error quantity is 0 (i.e., error absent data) is about 36%.
Actual measurement results confirmed that, when performing high speed playback with a digital VTR, the data of the portion above 50% of the approximate output during normal playback can be correctly restored by error correction. These measurement results were obtained with the rotary heads 8a and 8b tracking adjusted at the lower edge of the tape 9. For a detailed description regarding his point, refer to Okuma, et al., "Measurement of Error Incidents at High-speed Playback in Digital VTR", on 1991 Papers from the Convention (44th Convention) of Kyushuu Branch of Electrical and Related Institutes, page 158.
FIG. 50A shows the track pattern and the scanning traces of the two-channel rotary heads 8a and 8b during six-time speed search with the above-mentioned digital VTR. Since the rotary heads possess mutually different azimuth angles, the playback data from the channel rotary head 8a and 8b of the respective channels are as illustrated in the hatched portions of the figure. FIG. 50B shows, at the upper half, the playback signals obtained by the rotary head 8a. FIG. 50B also shows, at the lower half, the signals which are obtained as correct data through error correction. As indicated in the figure, in one rotary head scanning period, about 1/2 track (50%) of the video data is obtainable.
In the following description, it is assumed that, on the basis of the above results, if the amplitude of the playback signal at the output of the rotary head is 50% or more of the amplitude of the signal obtained at normal playback, the playback signals is restored as correct data through error correction.
Next is a description of a prior art recording format with reference to FIGS. 51A and 51B. FIG. 51A shows the video data of each field divided into 20 equal blocks according to their positions on the screen, with the blocks numbered according to the order of scanning. FIG. 51B shows the arrangement on the magnetic tape of the video data in each field divided into these blocks.
The blocks described above are termed recording format generating blocks, and are taken as units of recording format generation. In the actual VTR, shuffling is applied to each of the DCT blocks, but for case of description, the shuffling operation is not included in the following description.
FIG. 52A illustrates the track pattern and the scanning traces of the two-channel rotary heads 8a and 8b when performing six-time speed search with a digital VTR comprising the two-channel combination heads shown in FIGS. 46A and the above-mentioned recording format. Since the rotary heads possess mutually different azimuth angles, the playback data from the rotary heads 8a and 8b of the respective channels are as indicated by hatched portions in the figure.
The playback signal from rotary head 8a is indicated in FIG. 52B. FIG. 53A shows the signals output by the rotary head 8a and obtained as correct data through error correction during high speed playback.
In the following description, it is assumed that if the amplitude of the playback signals at the output of the rotary head 8a is 50% or more of the amplitude of the playback during normal playback, the playback signal is restored as correct data through error correction.
In FIG. 53A, o1, o2, . . . , o20 and e1, e2, . . . e20 correspond to the positions on the screen of each field illustrated in FIG. 51A. FIG. 53B shows the playback signals from a first track A1 (the track in which blocks o1 to o4 are recorded) of the odd field shown in FIG. 51B and synthesized by means of the field memory. The playback signals from the other 9 tracks are omitted. FIG. 53C shows the image obtained by synthesis by means of the field memory, on the basis of the video information (shown in FIG. 53A) of each field played back through the rotary heads 8a and 8B. FIG. 53C also shows the composite image obtained by synthesizing the video information of the two fields. The hatched areas in FIG. 53C correspond to the portions where the playback information is restored during six-time speed playback.
Thus, in a VTR possessing the prior art recording format, during six-time speed playback, playback data at certain positions within each block is completely unobtainable, as shown in FIG. 53C, even when the playback data is synthesized using a field memory. The video data at certain positions are therefore never rewritten or renewed, as shown in FIG. 53C, and the quality of the high-speed playback picture is not satisfactory.
FIG. 54A shows a two-channel opposing head arrangement on the rotary drum, which differs from the above-mentioned two-channel combination head arrangement. The recording track pattern formed on the magnet tape 9 is shown in FIG. 54B.
In the prior art example of FIG. 54A, pairs of two-channel rotary heads 8a and 8b of respective channels are provided adjacent to each other in a rotary drum 60, and are used for recording and playing back from information on a magnetic tape 9. During recording, the recording signals of two channels are substantially simultaneously recorded on the magnetic tape 9 by the rotary heads 8a and 8b (FIG. 54B).
For the purpose of the following description, the rotary head 8a is assumed to be one for channel A (CH--A) and the rotary head 8b is assumed to be one for channel B (CH--B). The CH--A rotary head 8a and CH--B rotary head 8b possess mutually different azimuth angles. The figure also shows the recorded tracks A and B formed by the rotary heads of the respective channels CH--A and CH--B. Also, in the prior art example in FIG. 54B, when two-channel opposing heads are used, with two-channel recording being performed by setting the drum rotation to 4500 rpm, one field of video data is divided among 5 tracks, i.e., one frame of video data is divided among 10 tacks.
Next, the description is continued assuming that the recording format used in this example is as shown in FIG. 51B, i.e., the same as the recording format used in the previous example in which the two-channel combination heads are used. FIG. 55A shows the track pattern and the scanning traces of the rotary heads 8a and 8b during six-time speed playback with a digital VTR using two-channel opposing heads. FIG. 55B shows the playback signal output pattern from the CH--A rotary head 8a.
FIG. 56A shows the playback output signal from the rotary head 8a during high speed playback, which can be restored as corrected data through error correction. The reference marks o1, o2, . . . , o20 and e1, e2, . . ., e20 correspond to the positions on the screen of the respective fields shown in FIG. 51A. FIG. 56B shows the playback signals blocks 1, 2, 3 and 4 within each field synthesized by using a field memory.
FIG. 56C shows the images obtained by synthesizing the video information of the respective fields obtained through the rotary heads 8a and 8b. A synthetic image obtained by synthesizing the video information of the two fields is also shown. The areas indicated by hatching in FIG. 56C correspond to the part where the playback information is obtained during six-time speed playback.
In the above manner, during six-time speed playback with a VTR using the prior art recording format, since playback data of certain fixed positions are not obtained at all, and the video data of the certain positions are not rewritten or renewed even when the playback data is synthesized using a field memory, so that satisfactory playback picture is not obtained.
Thus, it is seen from FIGS. 56A and 56B, that the use of two-channel opposing heads does not solve the problem in which the playback signals of certain positions are never obtained, and the picture of such certain positions remain unchanged.
In a conventional analog recording VTR for consumer use, by G4 head development, the video data within 1 scanning period can be played back from the track with nearly 100% efficiency. Also, since the video data of 1 field is recorded on 1 track, satisfactory high-speed playback is achieved using the playback signals which are obtained with nearly 100% efficiency within 1 head scanning period. In general, since the playback signal can be obtained with nearly 100% efficiency during 1 scanning period of the rotary heads, use of a field memory is not required for high speed playback.
On the other hand, with a consumer type digital VTR, the problem was encountered in that, since the data played back within 1 scanning period of the rotary head was about 50% as indicated in FIGS. 53A and 56A, playback efficiency was poor. Thus, when an integer multiple speed is used, the data of certain positions of the track was completely omitted, the data corresponding to the positions on the screen was never rewritten, and satisfactory high speed playback could not be achieved.